1. Field of the Invention
This invention relates to apparatus for monitoring the operation of heater controllers and, more particularly, to apparatus that determines if a heater controller is operating properly by monitoring the frequency at which it activates a heater.
2. Description of the Prior Art
In the prior art, it is common practice to use time proportioned control of heaters to maintain the temperature of a material they heat at a level that allows the material to be processed. Generally, a heater controller periodically activates the heater for a selected interval, and this periodic activation of the heater maintains the temperature of the material substantially at a desired temperature. More specifically, the heater controller sequentially applies pulses to the heater that have an amplitude, duration, and repetition rate determined by the characteristics of the heater. The pulse parameters are selected such that the periodic activation of the heater by the pulses results in the heater transferring enough heat energy to the material to maintain it substantially at the desired temperature. The pulses are generated by the operation of the control circuits in the heater controller which control the potential applied to a heater by its output stages.
The operation of such heater controllers is normally monitored by providing circuits in the heater controller for monitoring the operation of its control circuits. In this arrangement, an alarm is activated when the heater controller control circuits malfunction, indicating that the heater is not being properly activated. It is important that such improper heater activation be detected, since it can result in the material being heated becoming too cold, or too hot, for processing and this, in turn, can result in damage to the apparatus used to process the material. For example, if a failure in the heater controller control circuits results in no pulses being applied to the heater, the temperature of the material being processed may drop to a level where any attempt to extrude it will result in damage to the extruder screw. Conversely, if the switching circuit malfunction results in the heater being continuously activated, the temperature of the material may rise to a level that will damage the processing apparatus and also create a safety hazard.
While the prior art control circuit monitor is useful, it fails to give an indication of improper heater activation when a heater controller switching circuit is operating properly but, due to a malfunction in the output stage of the heater controller, the heater is either not being activated or is continuously activated. Obviously, failure to detect this type of malfunction can result in the same types of damage to the apparatus used to process the material being heated that result when a control circuit failure goes undetected. Furthermore, in order to add such a monitor circuit to a heater controller in which it was not originally included, circuits in the heater controller circuit must be modified. Such modifications can be relatively expensive, and they can also be a source of potential problems, since the heater controller circuit may be damaged during modification or as a result of improper modification.